KINETICS OF THE ASSOCIATION REACTIONS OF NO WITH THE SERIES OF CHLOROFLUOROMETHYL RADICALS CFXCL3-X

The kinetics of the reactions CF2Cl + NO + M --> CF2ClNO + M and CFCl2 + NO + M --> CFCl2NO + M have been investigated using pulsed laser ph otolysis/time-resolved mass spectrometry in the pressure range 1-12 To rr and at the temperatures of 233, 298, and 333 K, using N-2 as the bu ffer gas. The results, along with those obtained in preceding works fo r the homologous reactions CF3 + NO + M --> CF3NO + M and CCl3 + NO M --> CCl3NO + M (refs 5 and 7), were analyzed by a variational RRKM m ethod. This analysis shows that all kinetic, structural, and thermoche mical data of this series of species and of their corresponding reacti ons are internally consistent within the framework of the variational RRKM model used. By fitting the usual Tree's expression to the RRKM ca lculated rate constants values, themselves based on the experimental d ata, the broadening factor F-c approximate to 0.6 was obtained. Settin g F-c = 0.6 for all reactions, the following rate expressions have bee n obtained (units of cm(6) molecule(-2) s(-1) for k(0) and cm(3) molec ule(-1) s(-1) for k(infinity)): k(0)(CF3) = (2.0 +/- 0.4) x 10(-29)(T/ 298)(-3.2); k(infinity)(CF3) = (1.9 +/- 0.4) x 10(-11)(T/298)(-1.0); k (0)(CF2Cl) = (1.8 +/- 0.35) x 10(-29)(T/298)(-4.4); k(infinity)(CF2Cl) = (1.9(-0.5)(+1.0)) x 10(-11)(T/298)(-1.5); k(0)(CFCl2) = (1.0 +/- 0. 25) x 10(-29)(T/298)(-4.8); k(infinity)(CFCl2) = (1.9(-0.5)(+1.0)) x 1 0(-11)(T/298)(-1.5); k(0)(CCl3) = (0.85 +/- 0.2) x 10(-29)(T/298)(-5.5 ); k(infinity)(CCl3) = (0.55 +/- 0.12) x 10(-11)(T/298)(-1.6). It appe ars that in the low-pressure part of the falloff, the RRKM theory may be used in a predictive way for association reactions forming nitroso compounds: from the experimental determination of k(0), a reliable eva luation of the enthalpy of reaction can be obtained (within +/- 8 kT m ol(-1)), using beta(c) approximate to 0.2 and vice versa. The values o f k(infinity) have been found equal within uncertainties for those spe cies containing fluorine. This is to be compared to the almost equal e nthalpies of their corresponding reactions. In the case of CCl3, both k(infinity) and the reaction enthalpy have lower values than those obt ained for the other species of the series. In contrast, k(0) decreases regularly in the series, from CF3 to CCl3, and this trend is well acc ounted for by the calculations.